ARC IN AIR BETWEEN OTHER SUBSTANCES 71 



when the anode is iron, since there is more heat developed 

 near the anode with arcs in air than near the cathode. 

 That the conductivity of iron vapor is greater than that 

 of carbon is shown by the fact that the drop in potential 

 across an arc having both terminals iron is much less than, 

 that about an arc having both terminals carbon. 



This part of the explanation of the greater current from 

 iron to carbon was given by Arons, 1 but the difficulty of 

 keeping the iron hot enough to be the cathode must also 

 be considered. As will be shown in following paragraphs, 

 it is always necessary to have a point on the cathode at 

 a very high temperature. Manifestly, it is harder to keep 

 a spot on iron very hot than one on carbon, first be- 

 cause iron conducts heat more rapidly than carbon, and 

 secondly because iron melts and vaporizes more readily 

 than carbon, and heat is lost in producing these changes. 

 In order to furnish this heat the cathode drop must be 

 large. 



The need of this last consideration will perhaps be more 

 apparent in the case of discharge between a stationary 

 carbon rod and a carbon disk kept at a lower temperature 

 by rotating it. Here the current is the greater when the 

 moving carbon is the anode. Arons' explanation is not in 

 this case sufficient, since the vapor in the arc will be carbon 

 vapor, whichever way the current flows. On the other 

 hand, the second consideration applies, for it will clearly be 

 easier to keep the stationary carbon hot than the moving 

 one, so that the current will be larger when the stationary 

 carbon is cathode. 



A few observations made by myself on the cathode and 

 anode drops in a carbon-iron arc are quite in harmony with 



1 Wied. Ann., 57, 195; 1896. 



